Fuel Pressure

[Joel Russo]

So I dug out an extra Auto Meter liquid filled Pro Comp fuel pressure gauge out of the extra race car crap, and decided to mount it on the hood to monitor the pressure under boost.  I mocked up a plate, used some Pro Flo hose and Earl's fittings and made sure it didn't leak.  I have a small gauge on the rail that I have been using for years to set the pressure....Ap pears as though I have a discrepency between the two.
The rail mounted gauge reads 46 psi with the vac line off of the Accufab regulator.
At idle with the vac line on and me sitting in the car, the hood mounted gauge reads 52 lbs.  I ran the car a bit to make sure my pressure raises 1:1 under boost, and it does.  At 10 psi boost, pressure is 62 lbs.  Scanmaster tells me I'm at 780-800..
 
So, What gives??   
 
 

[earlbrown]

I took a look at the inside of an accufab regulator and I'd be amazed if any of them work correctly.

It's odd you're running more pressure than you should though.  I guess it's better than losing pressure under boost.

[Joel Russo]

I took a look at the inside of an accufab regulator and I'd be amazed if any of them work correctly.

You're the second person in two days that told me that about the Accufab...
So what regulator is the one to use?

[Steve Wood]

the rail mounted gauges are usually a p.o.s. when new and go down hill from there.  On the other hand, I would not be surprised to find the other gauge might be bad.  With gauges, quality is often reflected in the price.  52 does not sound right because with the hose on, engine running the pressure should be in the 30's.

Accufab is as good as any other as far as I have seen

[ttipe]

43-44 psi is where most (I think all) chips see static fuel pressure (idle rpm vacuum bias) around 700 rpm. 46 psi is static fuel pressure adjustment error (+2 psi) maybe. Your short term integrator should be driving learns closer to 128. With the static fuel pressure set to 44, pull the vacuum line off the regulator and your fuel pressure should be 38-39 psi. I wonder if you have a tear in the regulator diaphram or the spring is snagged/stuck. I wonder if a messed up return line could cause this?? 52 psi with the line off is not correct. The old KENNE BELL regulators seemed to work well and are meant to be rebuilt as necessary. If I sound like a nut case it's cause I am. Long day at work.     

[Joel Russo]

43-44 psi is where most (I think all) chips see static fuel pressure (idle rpm vacuum bias) around 700 rpm. 46 psi is static fuel pressure adjustment error (+2 psi) maybe. Your short term integrator should be driving learns closer to 128. With the static fuel pressure set to 44, pull the vacuum line off the regulator and your fuel pressure should be 38-39 psi. I wonder if you have a tear in the regulator diaphram or the spring is snagged/stuck. I wonder if a messed up return line could cause this?? 52 psi with the line off is not correct. The old KENNE BELL regulators seemed to work well and are meant to be rebuilt as necessary. If I sound like a nut case it's cause I am. Long day at work.   

Had some time yesterday so I ran the car a bit. 
Rail mounted gauge reads 46 psi with vac line off, and only drops to 45 with the vac line on.
Hood mounted gauge reads 52.5 psi with vac line off, and only drops to 52 with the vac line on.
 
My scanmaster mil vols read 780-800 under boost in third gear.
 
I have an old Kenne Bell fpr that was working fine when I took it off.  Should I buy a new Pro Comp fuel gauge for the hood and see what it reads, or replace the Accufab with the Kenne Bell and see what gives.  I'm thinking I'm in the correct range according to the scanmaster numbers, and probably have a bad gauge or two... 
 
 

[Steve Wood]

how much vacuum is the car making when idling?

[Joel Russo]

how much vacuum is the car making when idling?

I don't have a vacuums gauge, so how would I measure it?

[Steve Wood]

with a boost gauge that has vacuum on it, or a vacuum gauge.  Vacuum is one of the most useful readings you can look at to determine if the engien is okay, or there is something wrong with it.

If another gauge does not read properly, i would take the regulator apart and see if the diaphragm has a hole in it or something else wrong

[Joel Russo]

My boost gauge does not read vacuum.  I ordered a new Pro Comp liquid filled fp gauge today, so I'll swap it in and see what pressure it reads.  If it still reads high, I'll take the Accufab apart and inspect.

[Steve Wood]

When you set the fuel pressure with the hose OFF the regulator, it should normally read 43 psi for a TT chip.

When you put the hose back ON with the engine running, the vacuum normally in the hose will reduce the fuel pressure to something like 36 psi because the vacuum sucks up on the diaphragm and bypasses more fuel.

When the vacuum is reduced under throttle to zero while accelerating (just before boost registers), the fuel pressure should again be 43.

When the boost rises to say 10 psi, then the fuel pressure should read 53 psi as the diaphragm is being pushed down and less fuel is by passed.

How the fuel pressure can possible rise above the base setting when you install the hose at idle is beyond me because the vacuum in the hose should be dropping the pressure that the gauge sees.

First thing I would do is stick my thumb over the end of the hose and see if it is sucking against my thumb like it should be.  Second thing I would do is go by a cheap vacuum gauge at the store and stick it in the end of the hose to see exactly how much vacuum the car is making at idle to see if there is another problem with the car.  third thing I would do is to take the regulator apart and see if the diaphragm is holed or cannot move for some reason...but I already said that

[Forzfed]

My boost gauge does not read vacuum.  I ordered a new Pro Comp liquid filled fp gauge today, so I'll swap it in and see what pressure it reads.  If it still reads high, I'll take the Accufab apart and inspect.

It's good to read vacuum, because it will tell you if you hurt something.  I usually pull 18" of vacuum, when I blew my head gasket I pulled 0".

[PacecarTA]

43-44 psi is where most (I think all) chips see static fuel pressure (idle rpm vacuum bias) around 700 rpm. 46 psi is static fuel pressure adjustment error (+2 psi) maybe. Your short term integrator should be driving learns closer to 128. With the static fuel pressure set to 44, pull the vacuum line off the regulator and your fuel pressure should be 38-39 psi. I wonder if you have a tear in the regulator diaphram or the spring is snagged/stuck. I wonder if a messed up return line could cause this?? 52 psi with the line off is not correct. The old KENNE BELL regulators seemed to work well and are meant to be rebuilt as necessary. If I sound like a nut case it's cause I am. Long day at work.     

this is incorrect
static (or base FP) is the pressure with  line off  or as it would read at 0psi reference
line off should be 44 (varies with chip to achieve a blm around 128 ) and upon hooking back the vaccum line the pressure should drop
the amount of drop should be roughly 1/2 of the reading you would see on a vacuum gauge  because vacuum gauges read in In/HG and the conversion to psi is roughly 1/2
so if you have 18in/hg of vac on your gauge the pressure should drop 9psi (if satic was 44 new pressure should be 35) but dont expect that to happen on a stock return line with a hotwired  255lph pump

[ttipe]

If all is well and you remove the vacuum bias line from the regulator and the static fuel pressure should be 44.1 psi (with a 3 bar regulator). When I previously stated "static fuel pressure" I have regarded that to mean no vacuum/boost pressure influence on the regulator.Sorr y about the confusion. 

[Joel Russo]

A bit of a follow up on this issue.
 
I bought a new Fuel pressure gauge for the hood mount, and it reads 42 psi with the vac line off the fp regulator.  Looks like the old gauge was not accurate...  I also bought a vacuume gauge to see if I had poor vacuume.  I pulled the hose off the regulator and read 19" of vacuume.
 
When I put the hose back on the regulator, the fuel pressure only drops 1/2 to 1 psi.
 
PacecarTA said " so if you have 18in/hg of vac on your gauge the pressure should drop 9psi (if satic was 44 new pressure should be 35) but dont expect that to happen on a stock return line with a hotwired  255lph pump "
 
I have a Walbro 255, Hotwire kit, and stock return line... Am I good, or should I swap in a different regulator?